In this narrative, we have the typical hero's journey tale, where Arthur and Ford are called on by Slartibarfast to save the Universe. You can guess how well that goes. In between, they meet up with all their usual companions, encounter improbable circumstances, and repeatedly encounter many running jokes. On the whole the book maintains a brisk pace, the scenes work, the narrative works, and then you hit the end.
The end. There's the obvious end, where everything should have ended, and then there's the extended end, where, I suppose, Douglas hadn't written enough pages, so he tacked on a few useless chapters. These had the feel of a hurriedly written manuscript.
Aside from the end, the scenes and the jokes really go together well. Most of the the storytelling is solid, clear, and ridiculous in only the way that Douglas can make a story. I wish that I could start people with this Hitchhiker's, because it works far better than his first two books. While I have to say that it's less brilliant (but only in comparison to the radio shows), he more than makes up for that with engagement and a passel of jokes that works far better novelized than serialized.